As a result of sustained operations in Afghanistan and Iraq, there are an increasing number of U.S. military personnel and Veterans at risk of developing both substance use disorders (SUDs) and Post Traumatic Stress Disorder (PTSD). If left untreated, individuals with SUDs and/or PTSD are at risk for other mental health problems (e.g., depression), suicidal ideation and attempts, physical health problems, reduced resiliency, lost productivity, and family/relationship impairment. While mental health services are in place for U.S. military personnel, substantial gaps in the treatment of co-occurring SUDs and PTSD exist and there is little scientific evidence available to guide the provision of care. The proposed study directly addresses this knowledge gap by testing the feasibility and preliminary efficacy of an integrative behavioral intervention for the treatment of co-occurring SUDs and PTSD modified for use among U.S. military personnel (including National Guard and Reservists) who have served in Operation Enduring Freedom and/or Operation Iraqi Freedom (OEF/OIF). The intervention, called Concurrent Treatment with Prolonged Exposure or COPE, represents a novel treatment that integrates cognitive-behavioral therapy for SUDs with prolonged exposure therapy for PTSD. In earlier studies with civilians, COPE has demonstrated efficacy in reducing alcohol and drug use severity, PTSD symptoms, and associated mental health problems (e.g., depression, anxiety). In this hybrid Stage Ib/Stage II study, we will (1) use a manualized, well-tolerated behavioral treatment for SUDs and PTSD (COPE); (2) employ a two-arm randomized between-groups experimental design (COPE versus a modified treatment-as- usual (TAU); and (3) examine standardized, repeated dependent measures of clinical outcomes and process variables at 5 time points (pre-, mid-, and post-treatment; 3 and 6 month follow-up). The proposed project is directly responsive to the mission of the National Institute on Drug Abuse (NIDA) and the National Institute on Alcohol Abuse and Alcoholism (NIAAA) in that it seeks to enhance and accelerate research on the prevention and treatment of alcohol and drug use disorders (including illicit and prescription drugs) and comorbid conditions (e.g., PTSD, depression, sleep disturbances, HIV risk behaviors). The findings of this study will provide empirical evidence to inform policies and programs to better serve the needs of U.S. military personnel, Veterans, and their families.